This is missing any motivation for why you would want to actually
use this option. How about:
"In particular, you can use this to set driver properties for devices which
are created automatically by the machine model. (To create a device which is
not created automatically and set properties on it, use -device.)"
?
That's still not great, but I think it helps a little.
(ideally if -device/-global are the new standard interface we should have a
section explaining the general concepts and syntax and then documentation of
how to do specific things like networking via -device, and relegate all the
'legacy' options to a section clearly marked as 'legacy' with pointers back
to the new ways of doing the same thing. That would be a much bigger job,
though.)

Just while we're here, I'll be posting the following shortly:
commit 82aff428155d469ab705294486cc26cb34947999
Author: Anthony Liguori <address@hidden>
Date: Fri Dec 23 11:30:45 2011 -0600
qdev: don't allow globals to be set by bus name
This is technically a compatibility breaker. However:
1) libvirt does not rely on this (it always uses the driver name)
2) This behavior isn't actually documented anywhere (the docs just say driver).
3) I suspect there are less than three people on earth that even know this is
possible (minus the people reading this message).
So I think we can safely break it :-)
Signed-off-by: Anthony Liguori <address@hidden>
Regards,
Anthony Liguori